As a technical skill to improve the reception quality in a wireless communication system, there has been known heretofore a technique of transmission antenna diversity that transmits identical data from a plurality of antennas. Also known is a technique of closed-loop transmission antenna diversity where a base station decides the phases and amplitudes of signals outputted from a plurality of antennas in accordance with the data fed back from the receiver side.
In such a wireless communication system, the phase and the amplitude relative to each antenna where the received signal strength becomes maximum are detected from the propagation path characteristic obtained via each antenna and estimated on the receiver side (terminal device).
Subsequently the phase and the amplitude thus detected are quantized, and then the quantized data are sent to the transmitter side (base station).
On the transmitter side (base station), the phase and the amplitude of the signal to be transmitted from each antenna are adaptively controlled in accordance with the quantized data that represent the received phase and amplitude.
Such adaptive control is executed periodically and repeatedly on the transmitter side, so that the phase and amplitude of the signal to be transmitted can be optimized with respect to the propagation path characteristics that are fluctuated temporally, hence achieving improvement in the reception quality.
In order provide a better explanation of this wireless communication system, a concrete description will be given below of an exemplary case in which there are two transmitting antennas.
FIG. 10 is a block diagram showing a structure of a base station where a transmission antenna diversity is employed. In this diagram, the base station transmits a synchronous detection pilot signal and user data to a user.
The synchronous detection pilot signal transmitted to the user includes a first pilot signal (Pilot1) for transmission from an antenna 101 and a second pilot signal (Pilot2) for transmission from an antenna 102.
The first pilot signal and the second pilot signal are in an orthogonal relationship on the time base and are transmitted in a known symbol pattern, shown in FIG. 11, for example. The first pilot signal is diffused in a diffuser 103 into data of, e.g., a 4 MHz band in conformity with a diffusion code unique to the base station (sector), and then is supplied to a multiplexer 104. Similarly thereto, the second pilot signal is also diffused in a diffuser 105 into data of, e.g., a 4 MHz band in conformity with the same diffusion code as that used for diffusion of the first pilot signal, and then is supplied to a multiplexer 106.
It is supposed in this exemplary case that the user data includes two kinds, i.e., audio or other line switching data and packet data.
The line switching data are encoded in an encoder 107 for detecting and correcting any error in the wireless propagation path. Such an encoding process is executed by the use of, e.g., CRC (Cyclic Redundancy Check) in error detection, and a turbo code or a convolutional code in error correction.
The line switching data thus encoded are modulated in a modulator 108 by the use of BPSK, QPSK or QAM and then supplied to a multiplexer 109. Subsequently the line switching data are temporally multiplexed in the multiplexer 109 with a packet indicator that indicates, as shown in FIG. 12, the presence or absence of packet channel data and its rate.
The packet indicator may be mapped independently in some other channel by using another diffusion code.
The data thus multiplexed are diffused in a diffuser 110 by the diffusion code and the data channel identification code unique to the same base station, and then are supplied to a multiplexer 111.
Meanwhile, similarly to the line switching data, the packet data are also encoded in an encoder 112 and then are supplied to a diffuser 114 after being modulated in a modulator 113.
Here, as shown in FIG. 13, the packet data are discontinuous differently from the continuous line switching data. Therefore, in accordance with the presence or absence of the data and the packet data rate, the base station changes the value of the aforementioned indicator and inserts the changed value.
The diffuser 114 diffuses the modulated packet data by using the same diffusion code and packet data channel identification code as those used in the diffuser 103 and unique to the base station, and then supplies the diffused data to the multiplexer 111.
Subsequently, the multiplexer 111 forms antenna data by multiplexing the line switching data and the packet data diffused in the diffusers 110 and 114 by the respective identification codes, and supplies the antenna data respectively to antenna weight appliers 115 and 116.
The antenna weight applier 115 multiplies the antenna data by a coefficient for the antenna 101 and supplies the multiplied data to the multiplexer 104. The multiplexer 104 then multiplexes the multiplied antenna data with the diffused first pilot signal and supplies the multiplexed data to a transmitter-receiver 117. The transmitter-receiver 117 transmits the antenna data multiplexed with the first pilot signal to the user via the antenna 101.
The antenna weight applier 116 multiplies the antenna data by a coefficient for the antenna 102, and supplies the multiplied data to the multiplexer 106. The multiplexer 106 then multiplexes the multiplied antenna data with the diffused second pilot signal and supplies the multiplexed data to a transmitter-receiver 118. The transmitter-receiver 118 transmits the antenna data multiplexed with the second pilot signal to the user via the antenna 102.
The antenna data are thus transmitted from the antennas 101 and 102 respectively. In order to calculate the coefficients (antenna weight values) to be multiplied in the antenna weight appliers 115 and 116, the base station supplies, to an inverse diffuser 119, the data sent from the user and received by the transmitter-receiver 117 having a receiving function.
The inverse diffuser 119 executes a process of inverse diffusion of the data received from the user by using the diffusion code unique to the user, and then supplies the processed data to a demodulator 120. The demodulator 120 demodulates the data processed through inverse diffusion, and then supplies the demodulated data to an antenna weight data extractor 121. The antenna weight data extractor 121 extracts antenna weight data sent from the user per slot (e.g., per 0.667 msec), and supplies the extracted data to an antenna weight controller 122.
The antenna weight controller 122 maps the received bits and the values of the antenna weight data, and updates the antenna weight values of the antenna weight appliers 115 and 116 in accordance with the mapped values.
FIG. 14 shows an example of mutual correspondence between the received bits and the antenna weight values. This example assumes a case where 2-bit antenna weight data of “00” to “11” is transmitted per slot from the user's terminal.
For example, in a case where the antenna weight data “00” has been extracted, the antenna weight value (w1) of the antenna weight applier 115 is controlled to “1.0”, and the antenna weight value (w2) of the antenna weight applier 116 is controlled to “1.0+j1.0”. Similarly, in a case where the antenna weight data “11” has been extracted, the antenna weight value (w1) of the antenna weight applier 115 is controlled to “1.0”, and the antenna weight value (w2) of the antenna weight applier 116 is controlled to “−1.0−j1.0”.
When the output of the multiplexer 111 is “S”, the antenna weight value of the antenna weight applier 115 is “w1” and the antenna weight value of the antenna weight applier 116 is “w2” as mentioned, data of a value “w1*S” is outputted from the antenna 101 while data of a value “w2*S” is outputted from the antenna 102.
Supposing now that, as shown in FIG. 15, when the propagation path characteristics between the antennas 101, 102 and the user's terminal 130 are H1 and H2 (complex vectors) respectively, the data R received by the user's terminal 130 is expressed asR=(w1H1+w2H2)*S  Eq. 1
Next, FIG. 16 is a block diagram showing a user's terminal device to a base station where a transmission antenna diversity is employed. In this diagram, the antenna data transmitted from the base station via the antennas 101 and 102 are received by a transmitter-receiver 131 via the antenna 130 of the user's terminal device, and then are supplied to inverse diffusers 132 and 133.
The inverse diffuser 132 restores the antenna data by the use of a pilot data diffusion code and supplies the restored data to a pilot decoder 134. The restored data is a mixture of the data component obtained from the antenna 101 of the base station and the data component from the antenna 102, as represented by the following equations.P[n]=(AH1+AH2)  Eq. 2P[n+1]=(AH1−AH2)  Eq. 3
Therefore, the pilot decoder 134 estimates the propagation path characteristics H1 and H2 between the antennas 101 and 102 as follows to thereby calculate the estimated values α and β of the propagation path characteristics.α=H1=(P[n]+P[n+1])*A  Eq. 4β=H2=(P[n]−P[n+1])*A  Eq. 5
In calculating the estimated values of the propagation path characteristics, there may be a case of using the average of several samples so as to suppress any harmful influence of noises. The pilot decoder 134 supplies the estimated values α and β of the propagation path characteristics thus calculated to an antenna weight calculator 135 and a phase corrector 136 respectively.
On the basis of the estimated values α and β of the propagation path characteristics, the antenna weight calculator 135 selects, from the entire antenna weight values shown in FIG. 14, the antenna weight values (w1, w2) which are the most satisfactory to maximize the received data R in Eq. 1 (R=(w1H1+w2H2)*S). Then, the calculator 135 supplies 2-bit antenna weight data, which correspond to the selected antenna weight values, to an antenna weight data inserter 140.
The antenna weight data inserter 140 inserts the 2-bit antenna weight data through temporal multiplexing into the transmission user data encoded in an encoder 139, so that the transmission user data with the 2-bit antenna weight data attached thereto are transmitted to the aforementioned base station from the antenna 130 via a modulator 141, a diffuser 142 and the transmitter-receiver 131.
Meanwhile the phase corrector 136 calculates decode antenna data S according to Eq. 6 shown below on the basis of the antenna data restored in the inverse diffuser 133 (i.e., antenna data “R=(w1H1+w2H2)*S” in Eq. 1) and also on the basis of the estimated values α and β of the propagation path characteristics calculated according to Eqs. 4 and 5, and the antenna weight data w1 and w2 indicated to the base station.S=R*(w1α+w2)  Eq. 6
The decode antenna data thus calculated are demodulated in a demodulator 143 and are then decoded in a decoder 144 so that the decoded data are received as user data.
In the above description of the user's terminal device, the explained operation relates to the receiving circuit of merely one channel. When such reception is performed through a plurality of channels in the terminal device, a group of inverse diffuser 133, phase corrector 136, modulator 143 and decoder 144 may be provided for each of the channels, and the respective operations may be performed in parallel in the plural channels. In such a case, the diffusion code used for inverse diffusion is unique to each relevant channel.
The explanation given above is concerned with an exemplary case where the user's terminal device communicates with a single base station. There is also known a mobile communication system employing technology of site diversity (soft handoff) where, as shown in FIG. 17, identical data are transmitted from a plurality of base stations and are combined in a user's terminal device 130.
FIG. 18 is a block diagram showing the structure of a terminal device in a mobile communication system which employs such site diversity. The terminal device shown in FIG. 18 represents an example having two-channel receiving circuits 151 and 152 so as to be capable of communicating with two base stations.
In the case of this terminal device, there is included a combiner 153 which combines the decode antenna data obtained from the two-channel receiving circuits 151, 152 and then supplies the combined antenna data to a demodulator 143.
Also, in this terminal device, the receiving circuits 151 and 152 calculate estimated values α1, β1 and α2, β2 of the propagation path characteristics respectively between these circuits and the base stations in communication, and then supply the estimated values to an antenna weight calculator 135.
In the case of this mobile communication system employing such site diversity, the antenna weight values selectable by the terminal device need to be selected in consideration of the site diversity gain.
More specifically, it is necessary to select the proper antenna weight data values that are the most satisfactory to maximize the antenna data R calculated according to Eq. 7 given below.R=(w1(H1-1+H1-2+H1-3. . . H1-N)+w2(H2-1+H2-2+H2-3 . . . H2-N))*S  Eq. 7
Therefore, in the case of this terminal device, the receiving circuits 151 and 152 calculate the estimated values α1, β1 and α2, β2 of the propagation path characteristics respectively between the antennas. On the basis of the estimated values α1, β1, α2, β2 of the propagation path characteristics in the channels, the antenna weight calculator 135 selects the antenna weight values w1 and w2 that are the most satisfactory to maximize the received data R obtained according to Eq. 8 given below.R=(w1(H1-1+H1-2)+w2(H2-1+H2-2))*S  Eq. 8
A known diversity receiving device and a control method thereof are disclosed in International Pamphlet Laid-open No. 97/20400.
However, in a mobile communication system where data of plural channels are transmitted simultaneously from base stations to a user's terminal device as in a system employing site diversity such as W-CDMA (Wideband Code Division Multiple Access) or CDMA2000 for example, a channel with a site diversity gain and a channel without any site diversity gain may occasionally co-exist.
That is, in the mobile communication system mentioned above, as shown in FIG. 19, audio and other line switching data of a data channel 1 are transmitted simultaneously from two base stations to a user's terminal device, while packet data of a data channel 2 are transmitted from only one base station to the user's terminal device.
Consequently, there arises a problem that, if the data of the channel without any site diversity gain are received by using the antenna weight value calculated for the other channel with the site diversity gain, the receiving data characteristics of the channel without any site diversity gain are deteriorated. To the contrary, if the data of the channel with the site diversity gain are received by using the antenna weight value calculated for the channel without any site diversity gain, the receiving data characteristics of the channel with the site diversity gain are deteriorated.
The present invention has been accomplished in view of the problems mentioned above. An object of the invention resides in providing a wireless transmitter-receiver device that is capable of assigning an optimal antenna weight value to each channel of a terminal conforming to site diversity for communication with a plurality of base stations.